Mounting an airbag apparatus configured to absorb an impact applied on an occupant by inflating and deploying an airbag in a cabin in case of collision or sudden deceleration on a vehicle such as an automotive vehicle or the like is becoming popular. As such an airbag apparatus, various types such as a driver airbag apparatus mounted in the interior of a steering, a passenger airbag apparatus mounted in the interior of an instrument panel, side airbag apparatuses mounted in the interiors of side surface portions of the vehicle or in the interiors of seats, curtain airbag apparatuses mounted in the interiors of upper portions of doors, knee airbag apparatuses corresponding to knees of occupants, a pedestrian airbag apparatus mounted in the interior of a portion under a hood, and so on are developed and employed. Such an airbag apparatus generally includes a gas generator configured to generate gas under predetermined conditions, an airbag configured to be inflated and deployed by being connected to the gas generator, and a retainer configured to store the airbag. Also, an outer shell which forms the airbag might be formed with an air discharge port referred to as a vent hole. The vent hole has functions to prevent an internal pressure of the airbag from becoming excessively high and to alleviate the impact by discharging gas in the airbag when the occupant comes into contact with the airbag.
Here, FIG. 15 is a drawing for explaining an operation of a vent hole of an airbag apparatus in the related art, in which (A) shows a relationship between inflation and deployment time and an opening of the vent hole of the airbag, and (B) shows a relationship between a stroke and an impact absorbency of the airbag. A lateral axis of FIG. 15(A) shows time, and reference sign Te shows inflation and deployment completion time of the airbag, and reference sign Tf shows fully-open completion time of the vent hole. A vertical axis in FIG. 15(A) shows the opening of the vent hole, and 100% shows that the vent hole is a fully-opened state. A lateral axis in FIG. 15(B) shows the stroke (length in a thickness direction) of the airbag, and reference sign Se shows an inflation-and-deployment-completed state of the airbag. A vertical axis in FIG. 15(B) shows the impact absorbency of the airbag. The impact absorbency can be reworded as a force applied to the occupant when the occupant comes into contact with the airbag.
As shown in FIG. 15(A), the airbag apparatus in the related art can be identified into three types; Type I, Type II, and Type III according to the way to be brought into the fully-opened state (opening of the vent hole; 100%). The airbag apparatus of Type I is configured in such a manner that the vent hole starts to open simultaneously with the start of inflation and deployment of the airbag, and the vent hole is brought into the fully-opened state before the inflation and deployment completion time Te of the airbag. The airbag apparatus of Type II is configured in such a manner that the vent hole starts to open after some time from the start of inflation and deployment of the airbag, and the vent hole is brought into the fully-opened state at the inflation and deployment completion time Te of the airbag. The airbag apparatus of Type II is configured to solve a problem of the airbag apparatus of Type I such that a pressure loss is large since gas is discharged from the vent hole during the inflation and deployment of the airbag. The airbag apparatus of Type III is configured in such a manner that the vent hole is maintained in a substantially closed state (opening of the vent hole; about 0%) until the inflation and deployment completion time Te of the vent hole, and the vent hole is quickly brought into the fully-opened state substantially at the same time as the occupant comes into contact with the airbag. The airbag apparatus of Type III is configured to reduce the pressure loss more than the airbag apparatus of Type II. There exist the airbag apparatuses of Type III in various structures and, for example, the airbag apparatuses disclosed in Patent Document 1 to 3 are already proposed. In the airbag apparatus of Type III, it is considered to be preferable to reduce time Δt required for bringing the vent hole into the fully-opened state as much as possible, and it is actually designed so.
FIG. 15(B) is a drawing showing a relationship between the stroke of the airbag and the impact absorbency in the airbag apparatus of Type III. As shown in FIG. 15(B), in the airbag apparatus of Type III, since the vent hole is quickly brought into the fully-opened state as soon as the occupant comes into contact with the airbag, the impact absorbency demonstrates a maximum value Fmax in the vicinity of the inflation-and-deployment-completed state Se of the airbag, and then is lowered abruptly. In other words, the impact absorbency of the airbag apparatus of Type III assumes a substantially right-angled triangle.
Patent Document 1: US2006/0151979A1
Patent Document 2: US2005/0248137A1
Patent Document 3: JP-A-6-127330